Using stacked branches
======================

What is a stacked branch?
-------------------------

A stacked branch is a branch that knows how to find revisions
in another branch. Stacked branches store just the
unique revisions, making them faster to create and more
storage efficient. In these respects, stacked branches are
similar to shared repositories. However, stacked branches have
additional benefits:

* The new branch can be in a completely different location to the
  branch being stacked on.

* Deleting the stacked branch really deletes the revisions (rather
  than leaving them in a shared repository).

* Security is improved over shared repositories, because the stacked-on
  repository can be physically readonly to developers committing to stacked 
  branches.

These benefits make stacked branches ideal for various scenarios
including experimental branches and code hosting sites.


Creating a stacked branch
-------------------------

To create a stacked branch, use the ``stacked`` option of the branch command.
For example::

  bzr branch --stacked source-url my-dir

This will create ``my-dir`` as a stacked branch with no local revisions.
If it is defined, the public branch associated with ``source-url`` will be
used as the *stacked-on* location. Otherwise, ``source-url`` will be the
*stacked-on* location.


Creating a stacked checkout
---------------------------

Direct creation of a stacked checkout is expected to be supported soon.
In the meantime, a two step process is required:

1. Create a stacked branch as shown above.

2. Convert the branch into a checkout using either the ``reconfigure``
   or ``bind`` command.


Pushing a stacked branch
------------------------

Most changes on most projects build on an existing branch such as the
*development trunk* or *current stable* branch. Creating a new
branch stacked on one of these is easy to do using the ``push``
command like this::

  bzr push --stacked-on reference-url my-url

This creates a new branch at ``my-url`` that is stacked on ``reference-url``
and only contains the revisions in the current branch that are not already
in the branch at ``reference-url``.

If the local branch was created as a stacked branch, then you can
use the ``--stacked`` option to ``push`` and the *stacked-on* location
will be implicit. For example::

  bzr branch --stacked source-url my-dir
  cd my-dir
  (hack, hack, hack)
  bzr commit -m "fix bug"
  bzr push --stacked


Limitations of stacked branches
-------------------------------

The important thing to remember about a stacked branch is that the
stacked-on branch needs to be available for almost all operations. This is
not an issue when both branches are local or both branches are on the
same server.


Changing branch stacking
------------------------

Stacking of existing branches can be changed using the ``bzr reconfigure``
command to either stack on an existing branch, or to turn off stacking.
Be aware that when ``bzr reconfigure --unstacked`` is used, bzr will
copy all the referenced data from the stacked-on repository into the
previously stacked repository.  For large repositories this may take
considerable time and may substantially increase the size of the
repository.
